Chunk 14.0

In this chunk, the user requested implementation of three synthesis optimizations identified from the previous `perf stat` analysis: an arena allocator (Vec recycling pool) to eliminate the ~34% runtime spent on jemalloc alloc/dealloc in the `enforce` hot loop, interleaved A+B eval for better instruction-level parallelism, and software prefetch in the `eval` loops. The assistant implemented all three: added `zero_recycled`, `from_coeff_recycled`, and `recycle` methods to `LinearCombination` in bellpepper-core; added a `VecPool` to `ProvingAssignment` in bellperson that reuses 6 Vecs per `enforce` call; implemented `eval_ab_interleaved` that processes A and B LC terms in a combined loop with prefetch; and added `_mm_prefetch` intrinsics to the inner loops of `eval` and `eval_with_trackers`. The synth-only microbenchmark showed only a ~1% improvement (54.9s vs 55.5s baseline), far below the expected 15–25%. `perf stat` revealed instructions dropped 4.1% but IPC also fell from 2.60 to 2.53, indicating the interleaved eval's more complex control flow was hurting pipeline utilization. The assistant then reverted the interleaved eval back to separate `eval_with_trackers` calls while keeping the recycling pool and prefetch, to isolate whether the interleaving was the cause of the IPC regression. The chunk ends with a rebuild of this intermediate configuration, setting up the next iteration of measurement and refinement.

When Theory Meets Practice: The Disappointing Benchmark That Reshaped a Synthesis Optimization Campaign 1803 words

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